The optical properties of water--light scattering, light absorption, and light transmission--are determined by the nature of the suspended particles in the water such as particle concentration, particle size distribution, index of refraction of the particles, etc. If the nature of the particles does not change significantly in the water column, then there is good correlation between suspended particle concentrations and any of the optical properties. Since light scattering is relatively easy to measure it is widely used to measure suspended particle concentrations in water. Light absorption and light transmission sensors are by comparison relatively complex, large in size, and expensive to produce and for these reasons they are not as widely used as light scattering sensors to measure suspended particle concentrations in water.
Various prior art light scattering sensors have been designed to measure suspended particle concentrations in a fluid or the turbidity of the fluid. These sensors generally use a light source and a light detector appropriately arranged to measure light scattered by particles. However, the dynamic range over which a linear correlation has been obtained with these sensors has been limit to approximately 4 orders of magnitude. The linear measurement of the entire range of natural suspended particle concentrations that can exist in natural waters, which is greater than 6 orders of magnitude extending from approximately 1 microgram per liter (.mu.g/l) to greater than 1 gram per liter (g/l), has never been within the capability of a single design. An optical sensor design with the capability of linearly measuring this full range of suspended particle concentrations is highly desirable, especially if the device can be produced at low cost.
Prior art scattering sensors used to measure the turbidity of a fluid or the suspended particle concentration in a fluid generally fall into three categories: forward scattering, 90.degree. scattering, and back scattering. Forward scattering sensors have been used to measure low particle concentrations in a fluid but they are generally not capable of measuring high concentrations. Scattering sensors employing light scattering in the range of 90.degree. , commonly referred to as nephelometers, have been primarily used to measure the mid-range of suspended particle concentrations in a fluid. Back scattering sensors have been used to measure either low or high suspended particle concentrations, but none have been designed to measure both.
Forward scattering sensors measure light scattered at angles between 0.degree. and 90.degree. . Light scattered from low concentrations of natural particles in water is predominate in the forward direction, because of this forward scattering sensors have been used to measure low suspended particle concentrations in water. However, all prior art forward scattering sensors have used relatively long light path lengths (several centimeters or greater) which has prevented their use for the measurement of very high suspended particle concentrations. This is because the attenuation of light in water increases with both particle concentration and path length. Consequently, relatively long light path length sensors cannot measure high particle concentrations since very little light can reach the detector. Therefore the dynamic range of all prior art forward scattering sensors has been limited to the low end of the naturally occurring range of suspended particle concentrations. Examples of forward scattering sensors are disclosed in Simms U.S. Pat. No. 3,713,743 and Frungel U.S. Pat. No. 4,432,645; and in publications (1) Fry et al., "Mueller Matrix Measurements of Ocean Water", SPIE, vol. 489, Ocean Optics VII (1984 ), pages 127-129; (2) Hodara, "Experimental Results of Small Angle Scattering", AGARD Lecture Series No. 61 Optics of the Sea, 1973, pages 3.4-1 through 3.4-17; (3) Jerlov, "Scattering", Marine Optics, Elsevier Scientific Publishing Company, Amsterdam, 1976, pages 13-22, 33-43; (4) Petzold, "Volume Scattering Functions for Selected Ocean Waters", SIO, 1972, pages 3-13, 15-23, 25-27, 29-31, 33-36 and 77-78; (5) Spinrad et al., "Volume Scattering Function of Suspended Particulate Matter at Near-Forward Angles: A Comparison of Experimental and Theoretical Values", Applied Optics, Vol. 17, pages 1125-1130; (6) Thomdike, "A Deep Sea, Photographic Nephelometer", Ocean Engineering, Pergamon Progamon Press, 1975, vol. 3, pages 1-15; (7) Tyler et al., "Instrumentation for Measuring the Forward Scattering Coefficient of Sea Water", 1962, pages 393-395; and (8) Kullenberg, "Scattering of Light by Sargasso Sea Water", Deep-Sea Research, Pergamon Press, 1968, Vol. 15, pages 423-432.
Back scattering sensors measure light scattered at angles between 90.degree. C. and 180.degree.. The amount of light scattered at these angles is orders of magnitude less than at forward angles between 0.degree. and 90.degree. for low concentrations of particles in water. Because of this, in the past back scattering devices were primarily used to measure relatively high suspended particle concentrations in water. To increase the sensitivity of backscatter devices to permit the measurement of low particle concentrations in water more light must be radiated into the water and or higher detector sensitivity is required. A prior art back scattering sensor used to measured low particle concentrations has been developed by Moore et al., "Development and Use of Computerized Optical Sea-Truth Instrumentation for Lidex-82", SRI International, 1984, pages 1-66. This device has good sensitivity for the measurement of low particle concentrations but can not measure high concentrations of particles in water because it uses a relatively long light path length in water. Several prior art back scattering sensors have been used to measure very high concentrations of particles in water are exemplified by Downing U.S. Pat. No. 4,841,157, Shea et al. U.S. Pat. No. 3,665,201, Carr et al. U.S. Pat. No. 3,714,444 and Topol U.S. Pat. No. 3,586,862. However, these devices are limited in dynamic range because they do not have adequate sensitivity to measure low suspended particle concentrations in water.
Several other light scattering sensors designed to detect and or measure suspended particles in a fluid or the turbidity of a fluid have been patented or disclosed in the scientific literature. These include forward scattering sensors, backscattering sensors, fixed angle scattering sensors, scanning scattering sensors, and integrating scattering sensors.
None of the prior art light scattering sensors have been designed to combine the advantages of both forward scattering and back scattering in a single sensor design. Multiple scattering must be included as well since for very high suspended particle concentrations in water this is the major light scattering mechanism.
Further the optical design or optical components used or electronic design in the past has limited the performance of light scattering sensors. In general what prevents all prior art designs from achieving a wide dynamic range is a deficiency in one or more of the following parameters: the radiance distribution of the light source, the irradiance distribution of the light detector, the light path length in water, or the optical and or electronic signal to noise ratio. Because of this no prior art sensor exists that can measure the wide range of particle concentrations that can exist in natural waters, which is approximately 1 .mu.g/l to more than 1 g/l. Furthermore, most prior art scattering sensors are relatively large, complex, and expensive.